MCALLEN, Texas - Dozens of dirt-caked shoes popped out from beneath the silver Mylar blankets, where children lie on mats, watching cartoons, and parents cooed infants to sleep. Inside the chain-link pens of U.S. Border Patrol's largest holding facility, nearly 1,300 migrants were waiting Monday to be released, deported or transferred.

Set up in a converted warehouse during the 2014 child migrant crisis, the Central Processing Center was created as an overflow site for families and children. But it recent months it, too, has been stuffed beyond capacity. Derided as "la perrera" - "the dog kennel" - by migrants and border agents alike, it was the focus of public anger when photographs of children behind the chain links circulated last year and brought accusations of "kids in cages."

More waves of shock and anger at scenes of miserable, inhumane border conditions have followed, most recently last month when Vice President Mike Pence visited the McAllen border station nearby and saw nearly 400 men packed in a pestilent garage.

The Department of Homeland Security tightly limits media access and photography inside Border Patrol facilities, citing the privacy rights of migrants in its custody. But the restrictions have made it difficult for the agency to convince the public that the border is in crisis, and the Trump administration has allowed more video cameras and photographers inside its facilities, even though the images of detained children often generate anger and disgust.

Boys rest under Mylar blankets in a holding pen at the U.S. facility in McAllen, Texas, on Aug. 12, 2019.

CAROLYN VAN HOUTEN / THE WASHINGTON POST

The number of people in custody fluctuates daily - and sometimes hourly - at the processing center, as hundreds of thousands of adults and children from El Salvador, Honduras and Guatemala continue arriving at the U.S.-Mexico border despite the scorching summer temperatures. Movement is constant inside the rancid - though much improved - facility, with bus loads of immigrants being moved in and out of the border city.

Arrests along the Southern border have dropped 43 percent since May, when U.S. agents took 144,000 migrants into custody, the busiest month in a dozen years. But border-crossings are still at twice the level they were last year, and the tip of South Texas remains the busiest corridor. Nearly 37,000 people were apprehended in the Rio Grande Valley sector last month, U.S. data shows.

"We want to give folks a sense of what is going on down here," said Border Patrol agent Marcelino "Alex" Medina.

Inside the cavernous pair of warehouses in Southwestern McAllen, migrants are medically screened for common ailments and contagious diseases such as scabies, lice or chickenpox. Those needing medical help beyond basic services are sent to local hospitals, agents said.

Workers have access to face masks and gloves when entering one of two large containment areas, although the center is not immune from contagious diseases; the processing center had an outbreak of an influenza-like illness in late May that led Border Patrol to stop admitting people until the infections died down.

Once medically cleared, migrants are sent into holding pens. The center has seen tens of thousands of children and families since 2014.

Recently detained migrants, many of them family units, sit and await processing in the McAllen, Texas, facility on Aug. 12, 2019.

CAROLYN VAN HOUTEN / THE WASHINGTON POST

Recently detained migrants, many of them family units, sit and await processing in the McAllen, Texas, facility, on Aug. 12, 2019.

CAROLYN VAN HOUTEN / THE WASHINGTON POST

Unaccompanied children are separated by gender and kept in distinct pens, where they have access to crackers, juice and chips. A television runs programming for all hours except mealtimes, and they can choose to don provided sweatpants, T-shirts and shoes.

"Children are held on average about 26 hours in custody," said Oscar Escamilla, acting deputy Border Patrol agent-in-charge, who led a brief tour through the center. There were fewer than 100 unaccompanied children in Customs and Border Protection custody at the time of the tour on Monday - far from the peak a few months ago, when children were backed up in the immigration system and were crowded into the agency's facilities, sometimes for weeks.

During the tour, journalists were not permitted to talk to the migrants in custody, and most shied away from the cameras. Many retreated deeper within their pens and turned away.

Parents with children are held in separate enclosures, where dozens of men and women sat on metal benches or laid across gym mats on the concrete floor. Escamilla said migrants receive "shower wipes" or wet wipes when they first arrive, and they are permitted to take a shower within 72 hours.

Tired men bounced little boys on their knees, children munched on apples and others hid beneath blankets in the cell adjacent to a play area with a plastic playpen and a few toys. In one corner sat shelving units filled with clothes, baby formula, colorful toothbrushes and diapers.

Inside each section of cells, a guard monitors camera footage and keeps watch from a small tower elevated about eight feet from the ground. Escamilla said the agency chose chain-link fencing because it allows more visibility for agents and can help cut down on staffing needs.

Children and their fathers watch a movie at the processing center in McAllen, Texas, on Aug. 12, 2019.

CAROLYN VAN HOUTEN / THE WASHINGTON POST

Migrants can move freely within their respective holding pens, but unaccompanied minors, girls over the age of 10 and small children are assigned separate fenced-in areas. Between each holding area is a sanitation station containing about a dozen portable toilets and sinks that are cleaned twice a day.

There was no escaping the foul stench of days of accumulated dirt, sweat and waste - even with a far smaller number of detainees than when lawmakers visited the center in June and reported rampant overcrowding and horrible conditions.

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